A typical method for solvent purification and “moisture removal” is a thermal distillation. Since a solvent is very volatile, thermal distillation has a serious fire risk and a serious explosion risk. In addition, removing moisture from a solvent is a very inefficient method. Typically, distillation involves use of an appropriate drying agent. A typical drying agent may be Li, Na, K, CaH2, LiAlH4, or the like. These drying agents are very dangerous due to very high reactivity thereof. Over several years, many fires and explosions have been associated with thermal distillation of solvents. Chemists specify thermal distillation as one of the most dangerous processes among ordinary processes chemists use. Furthermore, maintenance of a distillation device and quenching of a drying agent is labor intensive.
Another purification method is a method using a so-called “Grubbs” device that uses a large solvent storage tank and an alumina/catalyst column so as to dry a solvent and remove oxygen to a desired degree of purity without using a thermo-reactive or water-reactive drying agent. The Grubbs device involves many empirical formulae and research space. The system uses a large storage drum accommodating 15 liters to 20 liters of a solvent and a column or a cylinder, which typically has a height of more than 10 inches and a diameter of more than 3 inches. A plurality of columns are used and connected in series. Each of the columns accommodates an activated filtering medium removing pollutants from a solvent when the solvent passes through the column. The solvent passes though the column only once to be filtered. When the solvent reaches a final column, the solvent is sufficiently filtered.
In order to exhaust all dangerous solvent fumes, a drum storage tank is typically filled with the solvent under a fume hood. The solvent may be supplied to a 4-liter container. Therefore, a plurality of containers should be used for filling the drum. The solvent may also be supplied to a large container such as a 10-liter container or a 20-liter container. Such a container commonly has heavy weight. In addition, it may be difficult or impossible to pour contents of the container into the drum storage tank under the fume hood.
On the other hand, purity of the solvent is a main point in a chemical industry handling a pure material such as fine chemicals. Typically, in order to purify a solvent during organic synthesis, distillation is performed several times, or moisture is removed by using a molecular sieve. In the case of distillation, in order to achieve the purpose of obtaining a pure solvent by using a boiling point difference, particle impurities and moisture may be simultaneously removed, but convenience of use is low and a freshly distilled solvent is difficult to instantly obtain as needed. The distillation is restrictively used so as to achieve the purpose of moisture removal by purifying a solvent by adding a molecular sieve to the solvent or allowing the molecular sieve to pass through a molecular sieve column, and generally, moisture may be removed to a level of 5 ppm.
Moisture in impurities included in a solvent is important in, in particular, carbonate series that are a main solvent of an electrolyte used in a lithium secondary battery. A carbonate-based solvent having high hygroscopic properties generally contains moisture of 200 ppm or more before molecular sieve purification, and contains moisture of 50 ppm after purification. Thus, when a lithium secondary battery is manufactured and operated, a battery swells or a negative or positive electrode material is deformed through a charging/discharging process. Therefore, capacity and a lifespan of the battery are decreased. Such a phenomenon is exponentially affected by a moisture content. Although the solvent purification and the moisture removal are important areas in fine chemicals, a conventional purification system has generally been used.
Recently, graphene has attracted attention as a candidate substance for developing a high performance material used in emitting heat and shielding an electromagnetic wave. It has been reported that the graphene had excellent properties due to excellent physical, chemical, and mechanical properties of the graphene, as compared to a material such as graphite or ceramic used in the past.
A graphene film may be produced through various methods. A method for producing a graphene film may be classified into a dry method and a wet method. As for the dry method, a graphene film is produced by forming a graphene layer by depositing graphene on a copper or nickel thin film in a single layer or several layers through vapor deposition, transferring the graphene layer formed on a metal thin film onto a plastic film substrate or the like by using a supporter such as Poly (methyl methacrylate) (PMMA) or the like, and removing the rear metal thin film through etching or the like. In the case of a chemical vapor deposition described above, there is environmental difficulty because graphene is formed in vacuum, a producing process is complicated, and a process of etching the rear metal thin film is necessarily performed for transparency of the graphene.
As for a general wet method, a graphene film is produced by preparing reduced graphene oxide, dispersing the reduced graphene oxide in a solvent, and coating the resultant solution on a target base material. In order to obtain a more uniform coating material than the produced graphene film, a graphene film is produced by coating graphene oxide on a film, drying the graphene oxide, and then, reducing the graphene oxide coated on the film by exposing the graphene oxide to a reducing agent gas such as hydrogen iodide (HI). In this case, since small graphene flakes are stacked through drying, it is difficult to exhibit basic physical excellence of the graphene.